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RE:Deflection criteria for veneer support

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The 0.3 inch deflection limit may actually be more rational than the L/600
criteria.  If we assume that there is a compensation joint at the bottom of
the beam you want to limit the deflection so that the joint will not close
up and cause the load to bear on the wall or glazing below.  Based on a 0.5
inch to 0.75 inch joint filled with caulking the limit of 0.3 inch seems
reasonable and is consistent with my practice of limiting deflection of
beams supporting the building skin to 0.25 inch in order to accommodate
0.5inch joints.

When we specify a deflection criteria in the form L/600 we are simply
limiting the maximum slope of the beam at the support.  This has very
little impact on the performance of the wall.  It would make more sense to
tie our criteria to the curvature of the beam or some other criteria
related to the damage in the veneer.  To the extent that criteria in the
form of L/600 was based on objective criteria, it is valid for a narrow
range of spans.

Where shear deflection is significantly larger than bending deflection then
the use of L/600 may make a lot more sense since you would in effect be
limiting the shear deformation.

It is interesting that the commentary t o ACI 530 makes it clear that the
deflection criteria only applies to unreinforced masonry.  It is assumed
that the reinforcing in the reinforced masonry would control the cracks. 
This assumes that the masonry then is self supporting.  I would still limit
the absolute deflection if I had a horizontal joint above or below the
wall.


Mark Gilligan

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